Constant Mole Fractions in Reacting Systems with a Phase Change

Undergraduate physical chemistry and chemical engineering thermodynamics/kinetics courses typically include methods and techniques for determining compositions of mixtures of chemicals in reacting systems. Students gain experience and understanding of these methods and techniques by solving specific example problems. A constructive educational approach may include finding a general solution to a special case and presenting an evaluation of the characteristics of the system. An example of a general solution and evaluation is presented in this article. The general solution is for a subclass of reacting systems comprised of the reactions in which one of the reaction products condenses. A special case of condensing systems is one where the mole fraction of one of the reacting species remains constant after condensation is initiated. The general solution for such a system is presented and the specific conditions required for establishing a constant mole fraction for one of the reactants for reactions conducted under constant total pressures are described. Such analysis is of significant practical value in determining conditions for efficient separation and recovery of products.